"Amp Tips" By Dan Torres "Clean up your act." (complete article available on our Literature page under "Vintage Guitar Reprints") This article will give you a few easy ways to make your tube amp cleaner sounding and/or modify the tone easily. Tube amplifiers can provide the absolute perfect clean tone for jazz, blues, Surf, rock, country, all kinds of music. It just takes some knowledge and the right approach. #1. Preamp tubes. It's pretty easy to clean up the amp and get a lot of very musical and unique tones by switching just a couple of tubes in the preamp. Normal guitar preamps use the 12AX7 (7025) tube as the main building block of the circuit. There are several other tubes that will plug into the socket with no circuit changes necessary. Each of them will change the gain and the tone of the preamp. Take a look at Chart #1 for some numbers, then a simplified explanation. Tube Gain 12AX7 = 100 5751 = 70 12AT7 = 60 12AZ7 = 60 6072/12AY7 = 44 5965/12AV7 = 27 12AU7/6189/5814 = 20 (Chart is used by permission of Torres Engineering Catalog #8) All of these tubes are "dual triodes," two circuits packaged in one tube. Therefore replacing a single 12AX7 with one of the alternates will change the gain of two stages of the preamp. To make it simple I will ignore the circuit losses and assume the maximum gain for each tube stage. With an input of .0001 volt two stages of a 12AX7 will boost the signal to 1 volt. But substitute a 6072 tube (available brand new now from the Torres Web Site) and the final gain is 0.1936. Only 19% of the gain of the 12AX7. This is much cleaner, less likely to distort. (Math .0001 X 100 X 100 again = 1 .0001 X 44 X 44 again - check out the chart = .1936) Application. Standard Fender two channel amps, like the Super Reverb, Twin Reverb, Deluxe Reverb, Vibroverb, Bandmaster etc. Change the second tube (V2) to a lower gain tube (this is the reverb/vibrato channel.) Stevie Ray Vaughan was very fond of the 5751 in his Super Reverbs for a cleaner tone. (Available brand new from the Torres Web Site.) Marshall JCM 800, JMP, Super Lead, Plexi, Fender Tweed amps like the Bassman, and Twin, try changing the second tube in these amps also. One channel Fender amps like the Princeton, change the first tube. See the "Tubes for your Amplifier" web page for all these tubes (the article continues with considerable technology and design explaination as "Clean Up Your Act" Vintage Guitar Reprint.) |
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